Aggregates and water -dispersible clay in Oxisols from southern Brazil

Abstract

Aggregation and dispersibility in water are two opposing behaviors of soil clay. Aggregates in Oxisols are very stable, but once broken, they release large amounts of water dispersible clay (WDC) that can then move with surface runoff. Understanding the behavior of WDC is important for proper management of Oxisols because agriculture is the major non-point source of water pollution in many areas. We sampled two Oxisols from southern, subtropical Brazil that were under three different land uses: native vegetation (NV), conventional tillage (CT) and no-tillage (NT). The mass of aggregates in five size classes ranging from 2000 μm to 53 μm in diameter was measured after 5 shaking times ranging from 0 to 27 h. The results were similar for both soils despite differences in texture. The majority of aggregates >500 μm broke down after 7.5 h shaking, releasing WDC but without producing smaller aggregates. Land use affected aggregate distribution in size classes, but not the aggregate stability of each class. Compared to the soil under conventional tillage (moldboard plowing and disking after each crop), the mass of 2,000–1,000 μm aggregates is about 2 times higher under no-tillage management and about 8 times higher under native vegetation. Aggregates in the 2,000–1,000 μm size class that remained after 27 h shaking in water have twice the carbon content as those that were not shaken. No such trend was observed in the microaggregates (106–53 μm). Kaolinite, hematite and maghemite were the main minerals present and there were no clear mineralogical differences in clay from microaggregates, macroaggregates, WDC, and bulk soil, and thus no support for the hypothesis that stable aggregates act to preserve specific mineral phases. X-ray diffraction data suggested less rutile and anatase in WDC and preferential loss of coarse kaolinite in the CT land use. The evidence suggests that organic matter is a major factor in the aggregate stability of these Oxisols, contradicting the conventional wisdom that iron oxide minerals alone account for the aggregate stability of Oxisols. ^